1. Field of the Invention
This invention relates to a radiation fin structure of a magnetron. The fin structure effectively radiates high temperature heat produced during oscillation of the magnetron as microwaves are generated.
2. Description of Related Art
Generally, a magnetron for generating microwaves, as shown in FIG. 1 of the accompanying drawings, is a type of diode which comprises an anode 1. The anode has a plurality of radially extending vanes 1a mounted on its inner periphery, and a direct-heated filament (referred to as a cathode) 4 disposed axially at its central position and surrounded by the anode.
In addition, the magnetron includes a magnetic circuit comprising upper and lower yokes 6a, 6b, upper and lower permanent magnets 7a, 7b attached to the lower surface of the upper yoke and the upper surface of the bottom of the lower yoke, respectively. Upper and lower magnetic poles 8a, 8b, act to apply a magnetic flux into an active space 5 defined between the anode 1 and the cathode 4. An output section, which is comprised of an antenna lead 9, an antenna seal 10, an antenna ceramic 11 and an antenna cap 12, emits microwave energy. The microwave energy is transferred from the anode 1 to the exterior of the magnetron, i.e., a cavity of a microwave oven, through a waveguide.
A plurality of a radiation fins 3 are parallel and vertically spaced-apart relative to one another, and are between the outer periphery of the cylindrical anode 1 and the inner periphery of the vertical wall of the lower yoke 6b. The radiation fins radiate heat at a high temperature, and the heat is generated from a collision between thermions, i.e. an electrically charged particle or ion emitted by a conducting material at high temperatures and the anode vanes 1a. There is disposed at an under side of the lower yoke a filter box 15 containing a choke coil 13 and a high voltage capacitor 14 which prevents unnecessary microwave components produced in the active space 5 from feeding back to the power source.
When cathode 4 is energized, thermions are emitted from the cathode into the active space 5 and effect cycloidal movement as they are subject to an electric field which is induced between the anode vanes 1a and the cathode 4. A magnetic flux is applied within the active space by magnetic poles 8a, 8b of the magnetic circuit. The thermions are accelerated and generate microwave energy which will be received by the anode vanes 1a.
When the thermions have reached the anode vanes, they retain the energy applied to them by the electric field. As a result, when they impinge against the anode vanes, the energy is converted into heat energy. In order to radiate the heat resulting from the impingement of the thermions against the vanes, the radiation fins 3, which are made of a heat conductive material of good quality, must be mounted externally of the anode 1.
In the past, in order to radiate heat of a high temperature generated during the oscillating operation of the magnetron, as shown in FIGS. 1 and 2, a plurality of plate type radiation fins 3 were fixedly mounted externally of the anode 1 in parallel, equally spaced-apart relation to one another. A blower fan (not shown) was mounted at one side of the electrical equipment chamber of the microwave oven to forcibly blow external cold air into the chamber. With this arrangement, when the external cold air is forcibly blown into the chamber by the fan, the blown air is guided to the yokes. The air then flows into the spaces between the radiation fins 3, thereby radiating heat from the fins.
When electric power is applied to the resonance section, i.e., the anode 1 of the magnetron, a given amount of microwave energy is produced within the section by movement of the thermions and transmitted to an output section. The remainder, which is referred to as anode loss, is converted into heat, and transferred to the fins 3, thereby being radiated to the exterior. Air which is blown by the fan flows between the spaced radiation fins 3 and between the lower yoke 6b (as shown in FIG. 2) and the radiation fins, thereby preventing a rise in temperature of the anode 1, or a lowering of the performance of the magnets 7a, 7b due to the temperature rise.
However, in the plate type radiation fins of the prior art which are generally equally spaced apart in relation to one another, as shown in FIG. 2, a separation phenomenon of an air stream occurs in the rear of the cylindrical anode 1 due to a difference in air pressure between the opposite sides and the back side of the anode. This takes place as the air passes through the gap between the adjacent fins 3 and around the cylindrical anode. The main stream of the cold air is excessively separated outwardly in the rear of the cylindrical anode by the separation phenomenon. As a result, since the cooling of the back side of the anode 1 by the main stream of the cold air is inferior to that of the front side, there is a great difference in temperature between the front and back sides. The output efficiency of the magnetron may be reduced and the thermal deformation of the vanes 1a due to the temperature difference at the anode may be increased, resulting in a shortening of the service life of the magnetron.